Hematopoietic cell transplantation for sickle cell disease: state of the art

Assessing Patient Preferences for Treatment Options for Pediatric Sickle Cell Disease: A Critical Review of Quantitative and Qualitative Studies

Sickle cell disease (SCD) resulting from a mutation of the β-globin gene results in sickle deformation of the red blood cell with consequent vaso-occlusion and intravascular hemolysis. SCD results in substantial morbidity, with impaired quality of life and premature mortality. Comprehensive and supportive care, disease modifying therapies and treatments with curative intent are each associated with asymmetrical costs, burden of care, and impact on survival and quality of life. There is thus a considerable decisional dilemma regarding treatment among patients and caregivers. The objective of this review is to evaluate the literature regarding quantitative and qualitative studies of patient preferences in therapy for SCD. Numerous survey-based studies have been performed evaluating SCD patients' treatment preferences. These studies are limited, however, as they are purely descriptive in nature with limited quantitative information on the relative value of treatment alternatives. Time trade-off and standard gamble studies and health state utility studies have also been utilized to quantify patient utility especially for curative hematopoietic cell transplant. However, these studies suffer from inaccurate assumptions regarding patient preferences. Qualitative studies have garnered the patient and caregiver perspective. Qualitative studies may be limited by selective and purposive sampling, and lack of representativeness due to sample size.

Curative Therapies for Sickle Cell Disease

Background: Sickle cell disease (SCD) is an inherited hemoglobinopathy associated with severe morbidity, impaired quality of life, and premature mortality. Hematopoietic stem cell transplantation (HSCT) is the only curative treatment available for patients with SCD and has a >90% event-free survival when a matched related donor is used. However, availability of human leukocyte antigen (HLA)-identical sibling donors for the SCD population is limited. The use of HLA-matched unrelated donors or related haploidentical donors has the potential to expand the donor pool. Methods: We reviewed the current literature on the indications for SCD transplantation, donor options, and the emerging use of gene therapy as a treatment option. Google Scholar and PubMed were searched using the terms SCD, bone marrow transplantation, donor sources, gene therapy, HSCT, and HLA matching. Additional articles were identified from the bibliographies of retrieved articles. All articles were reviewed for pertinent information related to SCD and transplantation. Results: HSCT has the potential to establish donor-derived normal erythropoiesis with stable long-term engraftment, amelioration of symptoms, and stabilization of organ damage. The majority of HSCT has been performed in children from HLA-identical sibling donors and has resulted in excellent rates of survival. The use of alternate donors such as HLA-matched unrelated donors and haploidentical donors has the potential to expand the applicability of HSCT for SCD. Early results in gene therapy for SCD are encouraging. Conclusion: Evaluation of the long-term benefits of curative therapies for SCD requires comparative clinical trials and studies of late effects.

Stem cell transplantation for treatment of sickle cell disease: bone marrow versus cord blood transplants

PURPOSE

The transplantation of stem cells harvested from bone marrow and cord blood for the treatment of sickle cell disease (SCD) is reviewed.

SUMMARY

Current treatment options have lengthened the lifespan of patients with SCD. Hydroxyurea is the standard of care for the management of SCD, but it does not prevent serious complications in all patients. For those patients with severe disease, stem cell transplantation may be an appropriate curative option. However, less than one third of these patients find an appropriate matched related bone marrow donor. Cord blood offers a more readily available source of stem cells for transplantation. Donor morbidity is eliminated, since the cells come from banked cords, and the harvesting process is noninvasive for the donor. Another advantage of cord blood transplantation is the lower occurrence of graft-versus-host disease (GVHD). One disadvantage of transplantation with cord blood includes delayed time to engraftment. Due to the mortality associated with stem cell transplantation, it may be most appropriate to reserve the procedure for patients who have a more severe course of SCD.

CONCLUSION

Although bone marrow, peripheral blood, and cord blood transplantation has been successfully performed in patients with SCD, data remain limited regarding the optimal preparative regimens, the most appropriate stem cell source, and the type of GVHD prophylaxis to be used after transplantation. More data are warranted before this treatment approach can be recommended as a standard of care for SCD.

Stable long-term donor engraftment following reduced-intensity hematopoietic cell transplantation for sickle cell disease

Reduced-intensity conditioning (RIC) regimens have the potential to decrease toxicities related to hematopoietic stem cell transplantation (HCT) in patients with sickle cell disease (SCD) and thus make HCT a more acceptable therapeutic option for this group of patients. We report the results of 7 patients enrolled on a study to evaluate safety and efficacy of HCT using bone marrow from an HLA matched sibling donor following an RIC regimen for patients with high-risk SCD. The conditioning regimen consisted of busulfan, fludarabine, equine antithymocyte globulin, and total lymphoid irradiation with shielding of the liver, lungs, heart, and gonads on day 1. Graft-versus-host disease (GVHD) prophylaxis consisted of cyclosporine and mycophenolate mofetil. The regimen was well tolerated, and all patients had hematopoietic recovery. Six of 7 patients are stably engrafted off immunosuppression and without sickle cell-related symptoms at 2 to 8.5 years after HCT. Consistent with the complete resolution of SCD related symptoms observed in the 6 engrafted patients, erythropoiesis of complete or predominantly donor origin was detected by red blood cell-specific chimerism assays, despite their having persistent mixed chimerism in the mononuclear and lymphoid compartments. These findings demonstrate the curative potential of allogeneic HCT after an RIC regimen in patients with SCD.

Sickle cell disease vasculopathy: a state of nitric oxide resistance

Sickle cell disease (SCD) is a hereditary hemoglobinopathy characterized by microvascular vaso-occlusion with erythrocytes containing polymerized sickle (S) hemoglobin, erythrocyte hemolysis, vasculopathy, and both acute and chronic multiorgan injury. It is associated with steady state increases in plasma cell-free hemoglobin and overproduction of reactive oxygen species (ROS). Hereditary and acquired hemolytic conditions release into plasma hemoglobin and other erythrocyte components that scavenge endothelium-derived NO and metabolize its precursor arginine, impairing NO homeostasis. Overproduction of ROS, such as superoxide, by enzymatic (xanthine oxidase, NADPH oxidase, uncoupled eNOS) and nonenzymatic pathways (Fenton chemistry), promotes intravascular oxidant stress that can likewise disrupt NO homeostasis. The synergistic bioinactivation of NO by dioxygenation and oxidation reactions with cell-free plasma hemoglobin and ROS, respectively, is discussed as a mechanism for NO resistance in SCD vasculopathy. Human physiological and transgenic animal studies provide experimental evidence of cardiovascular and pulmonary resistance to NO donors and reduced NO bioavailability that is associated with vasoconstriction, decreased blood flow, platelet activation, increased endothelin-1 expression, and end-organ injury. Emerging epidemiological data now suggest that chronic intravascular hemolysis is associated with certain clinical complications: pulmonary hypertension, cutaneous leg ulcerations, priapism, and possibly stroke. New therapeutic strategies to limit intravascular hemolysis and ROS generation and increase NO bioavailability are discussed.